The present invention relates to domain patterning and, more particularly, to domain patterning in ferroelectric materials via electric field poling.
Ferroelectric materials, by definition, have spontaneous polarization. That is, these materials have internal electric dipole moments. The direction of these moments can be controlled to form certain desired domain configurations within the ferroelectric. In this connection, much effort and research has been involved in developing structures having particular domain patterns. Reference is made, for example, to U.S. Pat. No. 5,036,220 issued Jul. 30, 1991 to Drs. Byer, Fejer and Lim relating to the creation of domain patterns within nonlinear materials for interaction with optical radiation to obtain frequency conversion via quasi-phasematching. (The term "optical" as used herein to identify EM radiation or EM radiation properties, is meant to define and encompass electromagnetic radiation in the visible wavelength spectrum and in other adjacent wavelength spectrums--typically radiation having a wavelength in the range of between 1 and 15,000 nanometers.) Much of this interest is due to the increasing possibility that frequency conversion via domain patterning will lead to quite reliable, inexpensive and small sources of desired radiation having adequate energy for its purposes. For example, much of the attention is devoted to generating "blue" optical radiation, i.e., radiation having a wavelength in the range of about 390-492 nm, to read compact discs.
One of the more significant approaches to domain patterning of a ferroelectric material, is the application of an electric field to the material to change the direction of spontaneous polarization in desired regions. This is commonly referred to as electric field poling. In this connection, ferroelectric materials are often sold in bulk form having spontaneous polarization in a single direction, e.g., the dominant spontaneous polarization extends throughout the material from one face to the opposite one. It is only necessary for quasi-phasematching with these materials to reverse the direction of the poling in specific regions. For most efficient quasi-phasematching, adjacent domains should have reversed directions of polarization with the width of each domain, i.e., the path therethrough of the expected optical radiation, being equal to one coherence length (preferably) or some other integer multiple of the coherence length through such material. (By "coherence length" is meant the distance over which the phase of the original optical radiation and the generated optical radiation slip by a factor of 180.degree.. The paper authored by J. D. McMullen entitled "Optical Parametric Interactions in Isotropic Materials Using a Phase-Corrected Stack of Nonlinear Dielectric Plates" appearing in the Journal of Applied Physics, Vol. 46, No. 7 (July 1975) provides a mathematical definition and treatment of coherence length.)
A major problem with electric field poling is that it is somewhat difficult to provide a high resolution domain wall between adjacent domains, i.e., the shape and location of the boundary between adjacent domains typically cannot adequately be controlled for at least some applications.